Additive Processes for Piezoelectric Materials: Piezoelectric MEMS

Abstract

Piezoelectricity has been underutilized in the MEMS world. Fabrication process compatibility, complexity, and the limited availability of repeatable and reliable piezoelectric thin films have limited the incorporation of piezoelectric thin films in MEMS. Advances in materials processing and a move toward system-in-package (SIP) concepts have pushed piezoelectric thin film devices toward mainstream acceptance. The advances in piezoelectric aluminum (AlN) thin films for film bulk acoustic resonators (FBAR) [1] have encouraged using piezoelectric transduction as an alternative to electrostatic actuation in RF (radio frequency) MEMS applications. The FBAR devices have tremendous performance advantages and smaller size over conventional bulk-machined quartz surface acoustic wave devices (SAWs) making them ideal for cellular phones. To date, FBAR devices have exceeded the performance of electrostatically driven MEMS resonator and filters operating in cellular phone frequency bands (nominally 0.7–6 GHz). In addition to FBAR technology, piezoelectric actuation/sensing is being examined for a host of applications including RF MEMS, small-scale robotics, resonant mass sensors, and energy harvesting [2, 3]. This chapter focuses on materials selection, material processing, and integration of piezoelectric thin films with conventional MEMS fabrication processes. The first sections introduce the reader to the fundamentals of both ferroelectricity and piezoelectricity. The remainder of the chapter addresses processing of the key ferroelectric and piezoelectric materials, namely lead zirconate titanate (PZT), AlN, and zinc oxide (ZnO).